1. Field of the Invention
The present invention relates to a ridge waveguide type distributed feedback semiconductor laser device and method for manufacturing the same.
2. Description of the Related Arts
A ridge waveguide type semiconductor laser device is a semiconductor laser device in which the stripe-like ridge makes it possible to confine electric currents and light and to accurately control the transverse mode of the laser operation. In a ridge waveguide type semiconductor laser device, the width of the stripe-like ridge (the ridge width) and the thickness between the active layer other than the stripe-like ridge region and the bottom surface of the ridge (the remaining thickness of ridge) determines how light is confined (or the broadening of light) and how electric currents are confined (or the broadening of electric currents), thus determining the device characteristics. Therefore, by accurately controlling the ridge width and the remaining thickness of the ridge, it is possible to increase the reproducibility of the device and to improve the manufacturing yield. By examining the electric current at threshold value of the laser device relative to the ridge width and the remaining thickness of the ridge, it has been found out that, in order to control the electric current at threshold value to be within .+-.5%, it is necessary to control the ridge width to be less than .+-.0.1 .mu.m and the remaining thickness of the ridge to be less than .+-.0.01 .mu.m.
Since the stripe-like ridge is formed by etching the portion other than the ridge, the etching depth determines the remaining thickness of the ridge. However, if the etching for forming the ridge is conducted by time control, it is difficult to control the remaining thickness of the ridge by etching, thereby decreasing the reproducibility of the characteristics. Therefore, a method is taken to stop the etching by using an etching stop surface and to accurately control the remaining thickness of the ridge by the etching, as is disclosed in Japanese Unexamined (Kokai) Patent Publication No. Sho 63(1988)-38279.
On the other hand, the distributed feedback semiconductor laser device (DFB-LD) is utilized as a coherent light source operating in a mono-axial mode and having a variable wavelength and a stable wavelength. There has been an increasing need of DFB-LD for use in optical measurement, optical communication/transmission, optical recording, laser beam printers, and the like. In DFB-LD, a diffraction grating is disposed on an active layer or on a guide layer. A distributed feedback of light is effected by the diffraction grating for operating the laser oscillation. By applying the above ridge waveguide to the DFB-LD, it is possible to accurately control the transverse mode of the laser operation. Therefore, DFB-LDs having a good reproducibility can be manufactured with good yield, so that the manufacturing method is highly valuable in industrial application.
Generally, an irregular (uneven) grating is used as a diffraction grating in DFB-LD. When a ridge is to be formed in DFB-LD, a grating is first formed, and a crystal growth is conducted on the grating. Subsequently, a ridge is formed by etching, as disclosed in Japanese Unexamined (Kokai) Patent Publication No. Hei 5(1993)-235463. FIG. 5 shows the structure of the distributed feedback semiconductor laser device thus manufactured.
The device shown in FIG. 5 is manufactured as follows. First, on an n-GaAs substrate 50, there are formed an n-Al.sub.0.6 Ga.sub.0.4 As cladding layer 51, an Al.sub.0.15 Ga.sub.0.85 As active layer 52, a p-Al.sub.0.5 Ga.sub.0.5 As carrier barrier layer 53, a p-Al.sub.0.25 Ga.sub.0.75 As guide layer 54, and a p-GaAs light absorbing layer 55 in this order by Metal Organic Chemical Vapor Deposition (MOCVD) method. Subsequently, a diffraction grating (optical waveguide stripe) having a pitch of 120 nm is carved on the p-GaAs light absorbing layer 55 by an ordinary two-beam interference exposure method and a wet etching technique. The diffraction grating has a depth of 30 nm. Afterwards, a p-Al.sub.0.75 Ga.sub.0.25 As cladding layer 57 and a p-GaAs contact layer 58 are formed by growing under a condition of substrate temperature being 750.degree. C., the V/III ratio in gas phase being 120, the growth speed being 25 nm/min, and the growth pressure being 76 Torr by using MOCVD method.
Then, a portion of the GaAs contact layer 58 other than the optical waveguide stripe is removed, and a portion of the cladding layer 57 is removed with use of an HF type etchant which selectively etches an Al-mixed crystal layer with a highly content of Al to form a stripe-like ridge having a width of 3 .mu.m. Subsequently, electrodes are deposited on the upper portion of the ridge and on the rear surface of the substrate, followed by cleavage to complete the semiconductor laser device. In forming the ridge, the etching stops at the p-Al.sub.0.25 Ga.sub.0.75 As guide layer 54/p-GaAs light absorbing layer 55 consisting of an Al-mixed crystal with a low content of Al which is hardly etched by the HF type etchant and having a grating formed therein.
In forming a ridge of DFB-LD, a grating is formed and a crystal growth is conducted on the grating, followed by forming a ridge, as shown above. However, in the conventional examples, the ridge is formed by selectively etching a portion of the p-Al.sub.0.75 Ga.sub.0.25 As cladding layer 57 with the surface in which the grating is formed being used as an etching stop layer. However, when a crystal growth is conducted on an irregularity such as a grating, the growth proceeds from various surfaces of the irregularity, so that growing layers from various surfaces overlap with each other at the portion where one surface is connected to another, namely, at the recessed and projecting portions of the grating. These overlapped portions generate a strain. A strain applied to the growing layer decreases the crystallinity. The etching rate increases in a layer having lower crystallinity when compared with a continuously grown layer having the same composition and no strain. In the above conventional example, since the etching stop surface has a grating, the etching rate near the etching stop surface of the p-Al.sub.0.75 Ga.sub.0.25 As cladding layer 57 is extremely greater than the continuously grown layer. Therefore, the vicinity of the etching stop layer is excessively etched, whereby a side etch is generated in the etching stop layer, making it impossible to obtain a desired stripe-like ridge. As a result of this, the reproducibility of the device decreases, thereby lowering the manufacturing yield.